Hitherto there have been disclosed the SBE (Supertwisted Birefringence Effect) type liquid crystal display devices which are known as a liquid crystal display device capable of providing a high contrast and large capacity display in multiplex driving at a high duty ratio.
However, the SBE type liquid crystal display devices are liable to have a displayed image and a background tinted with a specific color because the devices utilize a birefringence effect. This is liable to cause the following problems: (1) deterioration in display quality, (2) little flexibility to meet users' color requirements, and (3) difficulty in color display.
To solve a tinting problem and provide a clear black-and-white display, the following techniques have been proposed:
1. a dual cell mode where a dual cell is structured by disposing one liquid crystal cell on the other cell containing the liquid crystal layer twisted to the direction reverse to that of the liquid crystal layer in the former cell in order to prevent tinting by using the other liquid crystal cell as an optical compensating plate (Nikkei Micro-Device Oct. 1987 issue, pp. 84-88.), and
2. a phase plate mode where tinting is prevented by substituting a phase plate comprising a birefringent film for the liquid crystal cell of optical compensation used in the above dual cell mode.
In the above dual cell mode, a production cost and a weight of the device increase more than those of the device composed of a single liquid crystal cell and, because of the difficulty to efficiently make the thicknesses of both crystal cells uniform, it is hard to mass-produce the liquid crystal display devices having the uniform display characteristics.
In the phase plate mode, because of the difficulty to uniformly control the birefringence of a phase plate in a wide area, a color unevenness is liable to occur. Further, because of the difficulty to completely compensate the birefringence of a liquid crystal cell for display, tinting can not satisfactorily be prevented.
There has recently been the requirement for a large capacity liquid crystal display device having a large number of picture elements. In order to meet this requirement, it is necessary to drive a liquid crystal display device at a higher duty ratio, and so to sharpen an intensity variation of transmitted light through a liquid crystal cell to a driving voltage. To satisfy the above requirements, it is effective to make a pretilt angle and a twist angle larger.
In the above dual cell and phase plate modes, there is used a normally closed display mode where light is not transmitted when applying an OFF voltage and light is transmitted when applying an ON voltage, so that a larger pretilt angle is liable to increase leaked light in applying an OFF voltage and decrease the contrast of a displayed image, as shown in FIG. 1.
On the other hand, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 63-274927/1988 discloses a technique for providing a single liquid crystal display device of a normally open display mode with an improved achromatic property by optimizing a value of .DELTA.n.multidot.d and a setting angle of a polarizing plate. This technique, however, is liable to provide a darker display and a contrast not higher than 20 since .DELTA.n.multidot.d is set at a small value.